The invention relates to wire dot printers which print by a dot impact method.
Prior-art wire dot printers have a problem in that, upon continuous printing, print heads occasionally overheat due to heat generation from print head coils; this can lead to poor printing, component deterioration, and even component damage. To prevent this from occurring, control methods have been implemented by doing such things as installing a thermistor inside the print head and, in accordance with its output signal, performing "reduced-character-printing" and/or "printing-suspension" (i.e., temporarily stopping the print operation). By "reduced-character-printing," we refer to a printing process in which fewer (relative to normal printing, in which one row of characters is printed in one pass of a print head) print wires are driven and in which, for example, one row of characters is printed in one reciprocal pass (i.e., one forward pass and one backward pass) of the dot head. Here, we use the term "two-pass-printing" to indicate the printing of one row of characters by one reciprocal pass of a print head.
However, should the time that the printer is stopped be too long, a user may be displeased by the delay or start worrying that the printer is broken. In addition, there are times when reduced-character-printing is not sufficient to fully suppress a rise in print-head temperature. For these reasons, drive methods, like that shown in the flowchart of FIG. 1, have been developed which combine printing-suspension and reduced-character-printing. Here, FIG. 2 shows a graph of a change in print-head temperature T when controlled as in FIG. 1; and FIG. 3 shows the cumulative amount of printed characters W when controlled as in FIG. 1.
In the prior art, when a print-head temperature T is equal to or less than a previously set alarm temperature A, Step 21 ("S21" in FIG. 1; subsequent steps treated similarly) through Step 23 are performed repeatedly; and, as a result, normal printing is carried out. Normal printing is shown by, for example, the interval from time "0" to time "n" in FIGS. 2 and 3. During normal printing, temperature T gradually rises. When temperature T exceeds the alarm temperature A, the number of spontaneously driven print wires is halved and two-pass printing is begun. At Step 24, counting is started with a counter, hereafter referred to as the "C-counter," at the start of two-pass printing. Until a C-counter value, hereafter referred to as "count-value-c," exceeds a maximum-stop-time M at Step 25, two-pass printing is carried out at Step 26. Two-pass printing is shown by, for example, the interval from time "n" to time "p" in FIGS. 2 and 3. Should count-value-c exceed the maximum-stop-time M, or, in other words, should temperature T not become equal to or less than the alarm temperature A within a set time, the processing proceeds from Step 25 to Step 27; and printing is suspended. A printing-suspension is shown by, for example, the interval from time "p" to time "q" in FIGS. 2 and 3.
However, in this case as well, there is a fear that this delay may displease a user or make a user start worrying that the printer is broken. Furthermore, since the duration of the printing-suspension is long, one may have trouble obtaining sufficient printer throughput.
Moreover, with the dot print-head having multiple print wires, there is a well-known problem in that, because drive-coil magnetic-circuits of the individual print wires interfere with each other, as the number of print wires simultaneously driven increases, the energy required to drive each print wire increases. Consequently, as the number of driven print wires is increased, the amount of heat given off by each drive-coil also increases, as does the heat given off by the print head as a whole.